Solid state pilot devices made of sensors or transducers and active components ordinarily have required three wires, namely, a power supply wire, a signal output wire and a common wire, to connect such pilot devices to the control center. However, control system designers have been accustomed to using 2-wire pilot devices such as pushbutton switches, limit switches, pressure switches, etc., and for reasons of economy, prefer to keep the number of wires to a minimum when using solid state pilot devices rather than the ordinary mechanical types. It has, therefore, been found desirable to provide control means adapted to be operated by 2-wire, solid state pilot devices as well as by ordinary switches.